Question: Which of these numbers is prime? ${36,\ 51,\ 67,\ 75,\ 99}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18, and 36. The factors of 51 are 1, 3, 17, and 51. The factors of 67 are 1 and 67. The factors of 75 are 1, 3, 5, 15, 25, and 75. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 67 is a prime number.